1. Field of the Invention
The present invention relates to an imaging apparatus and an optical filter. More specifically, the present invention relates to an imaging apparatus that includes an optical low-pass filter disposed between an image sensor and a photographic lens to suppress a phenomenon of fake color that may occur due to a pseudo signal of an object image on an image sensor and relates to an optical filter.
2. Description of the Related Art
In a conventional imaging optical system for forming an object image on an image sensor such as a charge-coupled device (CCD) or a complementary metal-oxide semiconductor sensor (CMOS), a optical low-pass filter is generally used to restrict a high spatial frequency component of object reflection light and remove a color component different from the object reflection light that may appear due to a pseudo signal.
In this regard, an optical low-pass filter that utilizes a birefringent effect of a plurality of mutually laminated crystal plates is widely used. However, the anisotropy of a refractive index of a crystal plate is not so high. Accordingly, in order to obtain a predetermined spatial frequency to be restricted, it is generally necessary to use a crystal plate whose thickness is approximately 1 mm.
In a conventional method, an optical low-pass filter is constituted by three crystal plates in total, namely, two birefringent crystal plates and a λ/4-wave plate disposed between the two birefringent crystal plates. Furthermore, an infrared-light cut filter is bonded to the optical low-pass filter having the above-described configuration. The optical low-pass filter like this is fixed in a retaining frame. As a consequence, in the conventional method, the total thickness of the optical low-pass filter may become large. Therefore, it becomes difficult to reduce the size and weight of an apparatus. Furthermore, the optical performance may degrade due to increased aberrations or harmful phenomena such as ghosts.
On the other hand, a material having a high level of the anisotropy of the refractive index such as lithium niobate is known. When this material is used, since the anisotropy of the refractive index is high, thickness necessary to obtain the predetermined spatial frequency to be restricted can be far thinner than that in the case of a crystal plate.
However, in the case of using the lithium niobate, it is technically difficult to achieve an appropriately thin optical low-pass filter. Furthermore, it is technically difficult to appropriately laminate the components of the optical low-pass filter. As a result, manufacturing costs for the optical low-pass filter using the lithium niobate may increase.
On the other hand, Japanese Patent Application Laid-Open No. 08-122708 discusses a method related to a polymeric optical low-pass filter. More specifically, the method that Japanese Patent Application Laid-Open No. 08-122708 discusses is related to an optical anisotropic polymer film having a birefringent effect the same as a crystal plate.
Furthermore, Japanese Patent Application Laid-Open No. 10-186284 discusses a method for using a polyimide film having a depolarization effect instead of using a usual λ/4-wave plate which is constituted by crystal. By this method, a polarization state of a light beam emitted from a birefringent plate of the optical low-pass filter is converted into an approximately circularly polarized light beam.
Furthermore, acrylic resin, that is, an infrared light absorbing film that contains an infrared light absorbent has been marketed. The infrared light absorbing film has the optical performance equivalent to that of an infrared light absorbing glass.
The above-described film-like filter and the infrared light absorbing film are easy to manufacture. The optical low-pass filter including the infrared light absorbing filter using the above-described materials can be readily reduced in its size and weight. Furthermore, its manufacturing costs can be reduced.
However, the environmental resistance (in particular, the resistance to moisture) of the optical filter using the above-described resin film is low. More specifically, the resin substrate is easy to absorb moisture compared to a crystal material such as quartz. Accordingly, if a resin substrate is exposed to the air, the resin substrate absorbs moisture. Thus, the optical performance of a resin substrate may easily decrease and the optical performance of an optical filter like this can easily degrade.
In order to solve this problem, Japanese Patent Application Laid-Open No. 2004-301891 discusses a method for manufacturing an optical low-pass filter that has the improved environmental resistance. The optical low-pass filter discussed in the above Japanese Patent Application includes an optical anisotropic polymer film (the resin substrate) sandwiched between optical members such as a transparent glass or a infrared light absorbing glass having a high moisture resistance.
Furthermore, in the case where an anti-reflection evaporated film is coated on a surface of optical resin film constituting an optical filter, the adhesion of the evaporated film may be lower than the adhesion of a glass plate or a crystal plate. Accordingly, the evaporated film coated on a surface of the optical resin film may be easily delaminated.
With respect to an optical low-pass filter generated by a method discussed in Japanese Patent Application Laid-Open No. 2004-301891, the film surface is not exposed to the outside, therefore, it is not necessary to provide anti-reflection coating for a film plane. Accordingly, the method discussed by Japanese Patent Application Laid-Open No. 2004-301891 is effective in solving the above-described problem.
Generally, an array of conventional color filters corresponding to pixels of an image sensor is based on the Bayer array constituted by four pixels of RGBG. In this case, a phenomenon of fake color in a photographed image can be suppressed by employing an optical low-pass filter performing a four-point image division. In the four-point image division, a single spot light beam which is incident on an optical low-pass filter is divided into four points of light beams.
FIG. 7 illustrates an example of a common optical low-pass filter made from four laminated optical members to perform the four-point image division.
Referring to FIG. 7, a birefringent crystal plate 700 having a rotational angle of 0° is constituted by a birefringent material, such as crystal, and the birefringent crystal plate 700 performs a two-point division in a horizontal direction. An infrared light absorbing filter 702 is used for approximately matching a spectral sensitivity of the image sensor such as a CCD with a visibility of a person.
A λ/4-wave plate 710 is made of crystal. The λ/4-wave plate 710 is used for depolarizing object reflection light that has been linearly polarized by the birefringent crystal plate 700.
The object reflection light then transmits through a birefringent crystal plate 706c having the rotational angle of 90°. As described above, the point image division is performed in a vertical direction.
The four-point-divided object reflection light is finally incident on an image sensor 706. Thus, the fake color that may occur in a photographed image can be suppressed due to the low-pass effect.
Furthermore, a cover glass 711 is used to seal a chip portion 706a of the image sensor 706 in a ceramic package 706b. 
In the case of the above-described configuration, if the above optical anisotropic polymer film having the birefringence effect, a polyimide film having a depolarization effect, and an infrared light absorbing film having an infrared light absorption effect are used for the surface of the birefringent material 700, a sufficient environmental resistance cannot be achieved. More specifically, the surface of a resin film, whose environmental resistance is low, is exposed to the air. Therefore, in this case, the optical low-pass filter configured as above described may possess low reliability for an actual use.
Furthermore, since a film material itself is soft compared to crystal, a scratch or a defect may easily occur with respect to the optical low-pass filter having the above-described configuration. Accordingly, such optical low-pass filter requires careful handling.